validation/validator
An automated tool to check that coding used to create web pages is valid. It is important that coding is valid as this can impact on the accessibility of pages. Assistive technology used by disabled users such as screen readers may have problems if coding is invalid. Search engines may also have difficulty indexing pages. Most web editing packages include HTML validators or checkers, alongside spell checkers. Online validators are also available, eg W3C HTML validator and Style Sheet Validator.
Virtual host
A computer which can be forced to respond to multiple IP addresses and provide various services (typically different Web services) on each. Each of these IP addresess (which usually each have their own hostname) operate as if they were separate hosts on separate machines, although they are really all the same host.
Therefore, they are called "virtual" hosts. An example of virtual hosting is when an Internet Service Provider "hosts" World-Wide Web and other services for several customers on the same computer but gives the appearence that each of these services use separate servers.
Virus
A virus is a type of program that can replicate itself by making (possibly modified) copies of itself. The main criterion for classifying a piece of executable code as a virus is that it spreads itself by means of 'hosts'. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or carrying it on a removable disk. Additionally, viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with worms. A worm, however, can spread itself to other computers without needing to be transferred as part of a host. Many personal computers are now connected to the Internet and to local-area networks, facilitating their spread. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, and file sharing systems to spread, blurring the line between viruses and worms.
Viruses can infect different types of hosts. The most common targets are executable files that contain application software or parts of the operating system. Viruses have also infected the executable boot sectors of floppy disks, script files of application programs, and documents that can contain macro scripts. Additionally, viruses can infect files in other ways than simply inserting a copy of their code into the code of the host program. For example, a virus can overwrite its host with the virus code, or it can use a trick to ensure that the virus program is executed when the user wants to execute the (unmodified) host program. Viruses have existed for many different operating systems, including MS-DOS, AmigaOS, and Mac OS; today, the majority of viruses run on Microsoft Windows.A legitimate application program that can copy itself as a side-effect of its normal function (e.g. backup software) is not considered a virus. Some programs that were apparently intended as viruses cannot reliably self-replicate, because the infection routine contain bugs. For example, a buggy virus can insert copies of itself into host programs, but these copies never get executed and are thus unable to spread the virus. Self-replicating programs that have very limited spreading capabilities because of bugs should not be considered legitimate viruses.
Visits
a complete session of accesses to a certain web server conducted by one person. A visit is concluded when the customer hasn't viewed any page for a certain period of time (60 seconds in most cases).
VOIP
(Voice over Internet Protocol) The technology used to transmit voice conversations over a data network using the Internet Protocol. Such data network may be the Internet or a corporate Intranet.
VPN
(virtual private network) A private network constructed across a public network such as the Internet. A VPN can be made secure, even though it is using existing Internet connections to carry data communication. Security measures involve encrypting data before sending it across the Internet and decrypting the data at the other end. An additional level of security can be added by encrypting the originating and receiving network address.
